We all need help to succeed, whether it’s crucial advice from a seasoned professional, a testimonial about our product or service, or an introduction to an important contact. For better or for worse, sometimes it’s not what you know, but who you know. But how can you convince a busy and accomplished person who doesn’t know you that you’re worthy of their time?

Reaching out to someone you don’t know for help can seem very daunting. When I finished writing a book for McGraw-Hill, One Simple Idea for Entrepreneurs: Turn Your Dreams into a Licensing Goldmine While Letting Others Do the Work, they asked me to call other professional entrepreneurs and authors and ask them to review it. I didn’t know these people! It seemed like an impossible task. But I discovered how powerful leveraging fame and notoriety can be. It’s worth the discomfort. To be frank, it will open doors for you and help you sell product.

Here are three tips to help you reach out to people and get them to help you.

Successful people have gatekeepers. To get started, seek out those gatekeepers to find an appropriate email address or a telephone number (ideally, you’d have both). The internet has made detective work pretty easy, but don’t be afraid to pick up the phone and start asking, “How can I best reach him or her?” Publicists, public relations agencies, and publishers are great resources. Never succumb to rudeness or impatience: Your professionalism is your best asset. But you’ll also need to be persistent. Another important tip is to find and use people’s names. An email that starts with a vague “Hey” or is sent to a general inbox is very likely to be lost or ignored.

Use flattery. Everyone, including very successful people, loves to be flattered. But your flattery has to be sincere and come from the heart to be effective. When you reach out to someone, you’d better have done your homework. Why are they the right person to help you? What is it about them or their career that resonates so deeply with you? Know the details. When you send an email to that person, make it personal. Explain how this person has changed your life or inspired you. Successful people want to feel as though they’ve made a difference in people’s lives. But you must also keep it short. No one has the time or desire to read anything lengthy.

Timing is critical. Once you’ve opened the door to a conversation, let the person you’re reaching out to ask about you. The transition between introducing yourself and asking for a favor must be approached delicately. For example, when I reached out to some of my favorite authors to review my book, I told them that if it weren’t for them, I probably wouldn’t have written one at all. Then I told them it would mean a lot to me if they could read a chapter and tell me what they thought. Later I asked for a testimonial. When I reach out to successful CEOs, I tell them why I love their company and products and how their leadership and vision has inspired me. Then I ask for advice.

Please note that a 100 percent success rate is unachievable. I have been turned down many, many times. But I was able to get Mark Victor Hansen, the best-selling author of the Chicken Soup for the Soul series, and Barbara Corcoran, an investor and shark on the ABC reality television show Shark Tank, to vouch for me.

Reaching out to older, seasoned professionals who may have a bit more time on their hands and an appreciation for those who helped them in the past can be a good call. In a funny turn of fate, I now receive emails and calls from people who are hoping I can help them. And I can say that sincerity, flattery, and persistence really do go a long way.